In conventional earthworking implements, such as backhoes, excavators, loaders, and the like, an earthworking tool is mounted on one or more arms connected to a vehicle, and the tool and arm are moved by the extension and retraction of hydraulic cylinders. An operator controls the functions of the tool by the operation of levers associated with hydraulic valves. High pressure hydraulic systems are employed, in which pressures may range to 2000 pounds per square inch and more. Thus, depending on the diameter of the hydraulic cylinders employed and the leverage involved, great amounts of force can be brought to bear by the implement tool.
Hydraulic valves for such equipment are generally of the reversible on/off type wherein, for example, pressing a control lever in one direction opens a valve to cause a cylinder to extend; releasing the lever allows it to center thereby closing the valve; and pressing the lever in the opposite direction reverses the valve to cause retraction of the cylinder. The only input force required of the operator is that which is required to overcome a lever centering spring. Because the input force of the system is very low and the output force resulting therefrom is relatively high, the force or power gain of the system is very high.
The type of control systems used on conventional earthworking implements, absent the operator, is of the open loop type. That is, the output parameter of the system, displacement of the tool or arm, is not fed back to the system for comparison with the input parameter to derive an error signal, which then causes correction of the displacement until the error signal is zeroed. The only feedback available is visual feedback to the operator who makes a judgement about the displacement "error" and manually controls a corrective displacement. The lack of displacement or force feedback in conventional implement control systems results in a lack of "feel" in the control levers.
In general, each available function of the tool is controlled by a corresponding single axis control lever, although the combination of two functions on a dual axis lever, or joystick, is sometimes provided. In a conventional backhoe, four tool functions are controlled including boom swing, boom elevation, crowd or elbow angle, and bucket curl or pitch. Thus, a minimum of two dual axis levers or, more conventionally, four single axis levers are required to control the bucket of a backhoe. The two or four control levers do not resemble the configuration of the bucket arm such that learning to control a given backhoe by its levers is not intuitive. An operator must associate the labeled name of a lever or its position in relation to the other levers with the backhoe function it controls. An additional problem, particularly with four levers, is that efficiency of operation of the backhoe suffers form the need for the operator to switch hands from lever to lever to coordinate the movements of the bucket. As a result of these and other factors, considerable expense and practice time is often required to train a profficient and safe backhoe operator.
Because of the lack of feel in the hydraulic control levers, problems can arise even for an experienced operator. For example, if it is necessary to excavate near a pipeline or other existing structure using a backhoe, great car must be exercised to avoid damage to the existing structure, since resistance to movement of the bucket caused by contact of the bucket with the structure is not fed back to the control levers. In order to avoid such damage when working at close quarters, it is sometimes necessary to station an additional worker to observe the operation and signal movements to the backhoe operator.
It is foreseen that the use of a unilateral closed loop control system to control a backhoe might result in the creation of problems and hazards not present in conventional backhoe control systems. In a unilateral master-slave position control system, a position input at a control lever is compared with a sensed position of the corresponding function, and the function actuator, such as a hydraulic cylinder, is activated to reduce the position error sensed. Because it takes a finite amount of time to fill a hydraulic cylinder, it would be possible for the master lever to get considerably ahead of the cylinder controlled resulting in a desynchronization of the master and slave. The same result could occur, for example, if the bucket encounters an obstacle. The situation is further complicated by multiple control axes such that an operator can become, in effect, "disoriented" with respect to the positions of the control levers relative to the position of the bucket. Attempts by the operator and the control system to recover from such a situation could result in unpredictable movements of the backhoe arm. With a large heavy implement capable of applying tens of thousands of pounds of force in the vicinity of other workers, equipment, and structures, such a situation is of necessity to be avoided.
In conventional backhoes the bucket is movable through four degrees of freedom: extension and retraction, raising and lowering, swinging from side to side, and curl of the bucket about a horizontal axis. To some extent, this limits the utility of such a backhoe. In some situations, it would be desirable for the bucket to be able to yaw with respect to the dipper arm to which it is connected and to rotate about a wrist type of axis. This would provide the bucket with six degrees of freedom and would facilitate the use of a backhoe in trenching types of excavation in planes other than vertical. Such a capability might be desirable in excavating beneath an elongated horizontal structure, such as an existing pipe. One obstacle to developing such a six degree backhoe is that two additional single axis control levers or an additional dual axis control lever would be required which would exacerbate the problem of coordinating the control levers.